Basic hydraulic vehicle brake systems are equipped with a pedal-operated brake pressure generator and brake circuits with wheel brakes connected thereto. When the driver's objective is to initiate braking, the pedal is depressed, and the force applied to the pedal is transmitted to a master brake cylinder, boosted if necessary or desired. This causes development of a pressure in the brake circuits which becomes effective in the wheel brakes and results in a decrease of the rotating speed of the wheels with respect to their roll velocity. The result is that in the tire contact area of the wheels brake forces are transmitted which cause deceleration of the vehicle.
Wheel lock may occur in such a braking operation. Therefore, anti lock brake systems (ABS) have been developed which modulate the wheel brake pressures under braking conditions which cause the wheel to lose traction comprises pressure modulation valves, i.e., one inlet valve and one outlet valve, and the valve switching conditions dictate whether pressure fluid is removed from the wheel brakes for pressure reduction or pressure fluid is supplied to the wheel brakes for pressure increase. Further included is a pump which furnishes pressure fluid into the brake circuit in order to replace the pressure fluid removed for the modulation of the wheel brake pressures. The system may have many different configurations. Frequently, the so-called recirculation principle is used wherein the pump is configured as a non-self-priming return pump which returns the pressure fluid that was removed from the wheel brakes via the open outlet valve directly into the brake circuit upstream of the inlet valve. Locking of the wheels during a braking operation may effectively be prevented by appropriately activating the valves and the pump.
An anti-lock vehicle brake system of this type can be improved and extended to a brake system with driving stability control or traction slip control (DSC or TCS). In traction slip control operations, pressure is built up in the wheel brakes of the driven wheels, and the brake torque produced counteracts the drive torque so as to decrease the drive torque to an extent that the wheels do not lose traction. Thus, spinning of the wheels when starting to drive is prevented with this method. In driving stability control operations, a brake pressure (individual for each wheel) is built up on the wheels of the vehicle so that the brake forces produced generate a torque about the vertical axis of the vehicle which counteracts an excessive yaw rate of the vehicle.
These two types of controls and some other types of control not referred to have in common that a wheel brake pressure must be generated in individual or in all wheel brakes without pedal application being effected. Therefore, these braking operations are termed as independent force braking operations. In such braking operations, initially, the wheel brakes have to be filled with pressure fluid in a filling phase in order to generate a starting brake pressure which can then be modulated in a subsequent control phase in conformity with the respective control selected.
The pump of the anti-lock control system is used to build up the starting brake pressure. It has been shown that this pump is not in all cases capable of sufficiently quickly building up the required start brake value alone. So-called precharging pressure generators are installed into the brake system which, in the filling phase, deliver pressure fluid either directly to the wheel brakes (direct conduit) or to the suction side of the pump which conducts the fluid by increasing its pressure to the wheel brakes (indirect conduit).
The precharging pressure generator can be an additional pump as has been described in German patent application No. 42 13 710, for example. However, systems are also possible wherein the booster of the pedal-operated brake pressure generator is so activated that it operates the master brake cylinder even without pedal depression, and the hydraulic wiring of the brake circuits provides a connection between the master brake cylinder and the suction side of the pump. A system of this type is presented in German patent application No. 44 25 578, for example.
A so-called change-over valve is provided in both systems between the precharging pressure generator (pump or independently actuated master brake cylinder) to establish the connection at least in the control phase but also already in the filling phase of an independent cross-sections are used to this end, as described e.g. in German patent application serial No. 19529272455 dated Aug. 12, 1995.
In the filling phase, these valves provide a large cross-section after a short switching time so that the precharging pressure generator can deliver a sufficient amount of pressure fluid to the suction side of the (return) pump. However, the large cross-section suffers from the disadvantage that the pressure fluid is delivered to the pump in an undamped manner. This causes the development of loud noise because the pressure fluid column in the supply line to the pump is either greatly accelerated or abruptly slowed down with the opening and closing of the suction valve of the pump (configured as a piston pump). The pressure impacts in the suction line of the pump which are produced especially due to the abrupt slowing down are transmitted as structure-borne sound and emitted as air-borne sound transmission. Hence, major noises develop in an independent force braking operation which are possibly misinterpreted by the driver and, at least, are disturbing.
Therefore, an object of the present invention is to improve upon a vehicle brake system of the type described hereinabove so that an independent force braking operation is permitted at a minimum possible noise level. To this effect, the present invention proposes that there are three switching positions in the change-over valve, i.e., one closed, one open, and one throttled, and that the control device is so configured that in an independent force braking operation, the change-over valve is caused to adopt its throttled position in the control phase and its open position in the filling phase at least for one of the envisaged types of control.
However, even in controls which normally render necessary a rapid brake pressure build-up until the starting brake pressure, it will be sufficient under certain circumstances to effect the pressure build-up in the filling phase only by way of the direct conduit. This can be sufficient e.g. when braking on a low coefficient of friction, because the precharging pressure generator is then able to generate the starting brake pressure alone. The present invention further suggests that the control device includes a decision-making circuit which, on the basis of the data it has available, determines the starting brake pressure to be achieved in the filling phase and, when the latter pressure is lower than the initial pressure which can be generated by the precharging pressure generator, maintains the change-over valve in its closed position in the filling phase.
Another objective is to configure the change-over valve so that it is able to provide the required switching conditions in a most simple manner. The present invention discloses configuring the valve as a bistable valve which switches into the throttled or into the open position as a function of the inlet pressure while the actuating force remains equal.
A bistable valve of this type can be designed differently. In the present case, as proposed, the switching valve has two parallel arranged commutable passages, i.e., the passage of a pilot valve and that of a main valve, and the valve seat of the one valve is provided on the valve closure member of the other valve, and there is a joint actuation for both valves.
Further, it is disclosed that the valve closure member of the pilot valve is coupled directly to the actuating tappet, on the one hand, and the valve closure member of the main valve is coupled to the actuating tappet by way of a lost motion clutch, on the other hand. Valves of such a design are employed, for example, to permit quick opening of a main valve of large cross-section. This is because a certain pressure compensation is produced already by opening the pilot valve with a small cross-section so that the valve closure member of the main valve will put up only a small resistance to the actuating forces.
To achieve this object, the present invention discloses adapting the characteristics of a valve of this type (opening cross-sections of main and pilot valve, lost travels, spring forces and actuating forces) so that, when the valve is actuated under initial pressure, the actuating forces are not sufficient to overcome the pressure forces which act upon the valve closure member of the main valve, caused by the pressure gradient on the orifice of the pilot valve. The result is that the valve, switched under pressure, remains in its throttled position, and the throttling effect is determined by the switched orifice of the pilot valve.
As has been mentioned hereinabove, it may be necessary to switch the valve into the open position in the filling phase and into the throttled position in the control phase.
A switching signal sequence is provided in the control device to this end in order to initially render the open valve inactive again so that it closes to be subsequently re-actuated. The valve is moved into the throttled position (as explained hereinabove) when initial pressure is applied.